Monitoring leaf loss and possum impact on New Zealand beech mistletoes
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
Numerous anecdotal reports have suggested that possums are the primary cause of mistletoe decline throughout New Zealand, but little quantitative data has been collected to support this claim. The primary goal of this study was to quantitatively assess the annual amount of lea floss due to possum herbivory, insect herbivory, and leaf abscission on two populations of each of three endemic mistletoe species (Alepis flavida, Peraxilla colensoi, and P. tetrapetala) in four South Island beech forests (Craigieburn, Lake Ohau, Eglinton Valley, and Waipori Gorge). In all six populations, from February 1997 to February 1998, abscission accounted for by far the most leaf loss (range 10-84%, mean 33%), while insects and possums contributed to the mean loss of less than 3% of total leaf area across all six populations. Although possums and insects overall removed similar amounts of plant biomass, possum browse was significantly more heterogeneous than insect browse both on branches within a plant (possum c.v.=2.63, insect c.v.=1.94) and between plants in a population (possum c.v.=2.74, insect c.v.=1.17). Moreover, insects damaged all study plants but removed less than 9% of leaf area per plant, while possums only browsed 32% of the study plants but severely defoliated some plants. Mean leaf loss was significantly greater on plants browsed by possums (62%) than on unbrowsed plants (36%). Thus, while insect damage probably constitutes a predictable stress for most plants, possum browse is unpredictable and may cause a small increase in mortality of affected mistletoes, which could cause a slow decline at the population level for plants with long life-spans and slow reproductive rates. The second goal of this study was to compare the effectiveness of three commonly used methods of monitoring mistletoes: leaf mapping, visual estimates of plant health (i.e. percent browse scores, foliage density scores, and volume measurements), and photographs. Leaf maps provided the most detailed description of leaf loss but are too laborious to be used on a national scale. Visual estimates were subjective, did not correlate to quantitative leaf map data, and failed to detect the loss of branches. Foliage loss estimated from an examination of repeated photographs corresponded extremely well to leaf loss estimates from leaf maps. A monitoring protocol is suggested that includes photographing mistletoes every winter and visually estimating possum browse each winter and summer.